Polytetrafluoroethylene (PTFE) yarns are superior in terms of heat resistance, weather resistance, chemical resistance, low friction coefficient, low dielectric constant, and biocompatibility (anti-thrombic properties). Accordingly, these yarns have been used in a broad range of fields. For example, such yarns have been used as supports in various types of membranes such as reverse osmosis membranes, ultra-filtration membranes, precision filtration membranes, filtration membranes for bag filters, power filter filtration membranes, and ion exchange membranes. There are also cases in which such yarns are used in surgical sutures and surgical application cloths.
Since melt molding is difficult in the case of (PTFE), such yarns are ordinarily manufactured by emulsion spinning methods and slit yarn methods. Among these methods, slit yarn methods are preferred in that there are few impurities, and such yarns are especially ideal for use as supports in ion exchange membranes.
In slit yarn methods, yarns are manufactured substantially as follows: specifically, a PTFE powder is first mixed with a lubricant to form a paste. This is extrusion-molded and dried by heating so that the lubricant is removed. The extrusion-molded product thus obtained is drawn at a temperature below the melting point of the PTFE to form a sheet. Then, this sheet is slit in a long slender configuration to form a tape. A slit yarn is then manufactured by drawing this tape at a temperature equal to or greater than the melting point of the PTFE.
However, the abovementioned slit body of the sheet (tape) is extremely flexible, so that the tape may be bent in the longitudinal direction while running through the apparatus. Furthermore, the bent width in the direction of length of the tape is non-uniform, so that there is a large variation in the diameter of the yarn.
Furthermore, if the tape is made thinner, the abovementioned bending can be reduced; however, the softness of the slit yarn obtained drops, and the slitting workability also drops.
Furthermore, in the slit surface of the slit body (tape), the micro-fibers (fibril portions forming the porous structure) are cut so that nap tends to be formed, thus generating fiber debris, and forming sites for yarn breakage.